Times are m:ss after the origin time. Faint = wave arrives but is below the noise floor for this magnitude. “—” = in a shadow zone.
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Synthetic seismogram (log-like amplitude scale). Click a station on the globe or in the table to change it.
First-arrival travel-time curves computed by tracing rays through the layered model. Dots are the stations; the horizontal line is the simulation clock.
What is an earthquake?
Rock on either side of a fault is slowly squeezed, stretched or sheared by moving tectonic plates. Friction keeps the fault locked until the stored elastic energy exceeds its strength. Then the rock slips in seconds to minutes and radiates energy as seismic waves.
The hypocenter (focus) is where the rupture starts underground; the epicenter is the point on the surface directly above it. The glowing orange lines on the globe are plate boundaries, where most earthquakes occur — especially the Pacific “Ring of Fire”.
P-waves — primary, compressional
Particles move back and forth along the direction of travel, squeezing and stretching the rock like sound in air or a slinky pushed end-on. P-waves are the fastest (≈ 5–8 km/s in the crust and upper mantle, up to ≈ 13.7 km/s at the base of the mantle) so they arrive first. They travel through solids and liquids, so they pass through the outer core — but bend sharply on entering it.
S-waves — secondary, shear
Particles move perpendicular to the direction of travel, shaking the rock side-to-side like a rope flicked sideways. S-waves are about 60 % as fast as P-waves (≈ 3–7 km/s) and carry more energy, so they shake the ground harder. A liquid cannot sustain shear, so S-waves cannot cross the liquid outer core. That is how Richard Oldham and Beno Gutenberg first proved the core is liquid.
Surface waves — Love & Rayleigh
When body waves reach the surface they excite waves that travel along it. Love waves shake the ground horizontally; Rayleigh waves roll it in an ellipse like ocean swell. They are the slowest (≈ 3–4 km/s here modelled as 3.5 km/s) but are usually the largest and most destructive, and they fade quickly with focal depth — try a deep-focus preset and watch the golden ring almost disappear.
Inside the Earth: layers & speeds
| Layer | Depth (km) | State | P km/s | S km/s |
|---|
| Crust | 0 – 35 | Solid | 5.8 – 6.8 | 3.2 – 3.9 |
| Upper mantle | 35 – 670 | Solid, plastic | 8.1 – 10.3 | 4.5 – 5.6 |
| Lower mantle | 670 – 2,891 | Solid | 10.8 – 13.7 | 6.0 – 7.3 |
| Outer core | 2,891 – 5,150 | Liquid | 8.1 – 10.4 | 0 |
| Inner core | 5,150 – 6,371 | Solid | 11.0 – 11.3 | 3.5 – 3.7 |
This simulator uses a simplified, PREM-inspired radial model. Real Earth has more detail — oceanic crust, a low-velocity zone, slabs and plumes.
Refraction & shadow zones
Wave speed generally increases with depth, so ray paths curve back toward the surface (Snell’s law in a sphere — see the curved lines in the cross-section). At the core–mantle boundary P-waves slow from ≈ 13.7 to ≈ 8 km/s and bend inward, leaving a P-wave shadow zone roughly 100°–140° from the epicenter where no direct P arrives. S-waves stop altogether, so there is an S-wave shadow beyond ≈ 100°. Inge Lehmann used faint P arrivals inside the shadow to discover the solid inner core in 1936.
Try it: trigger a quake, then compare stations at 60° and 130° in the Stations tab.
Magnitude, energy & intensity
Magnitude measures the energy released at the source. Each whole step means ≈ 10× larger ground motion and ≈ 32× more energy, so an M9 releases about 1,000 times the energy of an M7. Intensity (Modified Mercalli, I–XII) measures the shaking at a place, which depends on distance, depth and local soil. The Event-facts panel shows an estimate for the epicenter.
How is an epicenter located?
P-waves outrun S-waves, so the delay between them (S–P time) grows with distance — roughly 8 km of distance per second of delay for nearby events. One station gives only a distance (a circle around it); three or more stations let you triangulate where the circles intersect. Pick different stations and watch S–P grow in the Stations tab and on the seismogram.
Experiments to try
- Slide the depth from 10 km to 600 km: surface waves collapse, body waves barely change.
- Compare M5 and M9: which stations still detect the P-wave?
- Place the epicenter opposite a station: what arrives at the antipode, and when?
- Switch to Cutaway and watch the wavefronts bend through the mantle and into the core.
About this simulation
Rays are traced in 2-D through a spherical, radially symmetric model with the ray parameter conserved. The wavefronts are then revolved around the source axis. Surface waves use a constant 3.5 km/s, amplitudes are a toy model, and reflections, conversions, attenuation and 3-D structure are ignored — it is a teaching tool, not a research code.